The problem with "Open Source" is that we can't know what the author meant by it. It's not a question of ethics at this point, it's a question of precise language. That he used "freely licensed assets", which could mean any number of things, makes it obvious that he was just very sloppy.
He could have used any of the definitions of FLOSS that are available and at least "copyleft or public domain" for the assets or whatever. He didn't. We can't know what he meant.
The term "Open Source" is very clearly defined: https://opensource.org/osd
Unless you have some reason to assume that somebody is using this term mistakenly (which may happen with any term), there's no problem with it.
Actually, "copyleft or public domain" is a bad term, because it would exclude things like Godot Engine, which is neither copyleft nor public domain (it's MIT). Don't use terms like that, use the clearly defined ones like "open source", "free software" or "FLOSS" - they all mean the same.
